The invention relates to a hydraulic system for an automatic transmission, in particular a dual-clutch transmission, of a motor vehicle, a motor vehicle with such a hydraulic system and a method for operating such a hydraulic system.
In a dual-clutch transmission gears can be changed fully automatically by means of two sub-transmissions. Torque transmission is accomplished via one of two clutches, which connects the two sub-transmissions with the drive. The clutches and the actuators for engaging the gears can be hydraulically actuated by means of a hydraulic system.
From DE 10 2011 100 836 A1 a generic hydraulic system is known which forms the starting point of the present invention. The present invention relates in particular to program modules, which are required for a signal processing of the actual current consumption of the pump drive in the hydraulic system. By means of this signal processing it is recognized whether or not a requirement for charging the pressure accumulator exists.
The hydraulic system known from DE 10 2011 100 836 A1 is divided into a high-pressure circuit and a low-pressure circuit. In the high-pressure circuit a pressure accumulator is provided with which a strongly temperature-dependent accumulator pressure can be provided in the range of for example about 30 bar. In addition the hydraulically actuatable clutches and actuators (for example gear shift elements) are arranged in the high-pressure circuit. On the other hand the low-pressure circuit operates at a hydraulic pressure in the range of for example about 5 bar. The cooling of the sub-transmissions is accomplished by means of the low-pressure circuit via the hydraulic fluid.
In the generic hydraulic system the low-pressure circuit has a cooling hydraulic pump and the high-pressure circuit has a charging hydraulic pump with which the pressure accumulator is charged to the required accumulator pressure. The two hydraulic pumps are driven via a common drive shaft by means of a shared electric motor. The electric motor is controlled by a control device. When recognizing a requirement for charging the pressure accumulator, the electric motor is controlled to run at a charging setpoint speed. As an alternative and/or in addition the electric motor is controlled to run at a cooling setpoint speed when for example recognizing a requirement for cooling. In addition the high-pressure circuit and the low-pressure circuit can be connected via a bypass line with integrated control valve. The control valve can be switched between a charging position and a non-charging position (cooling position) without external energy input, i.e., automatically, in dependence on the accumulator pressure in the high-pressure circuit. In the charging position the hydraulic system operates in the charging mode (i.e., the charging hydraulic pump is fluidly coupled with the high-pressure circuit) and at high pump load with correspondingly high actual current consumption. In contrast, in the non-charging position (i.e., in the cooling position) of the control valve, the hydraulic system operates for example in a cooling mode or in another mode for example a filter-cleaning mode.
For reasons of clarity the following discussion mostly refers to the cooling position of the control valve. It is noted that the cooling position and the non-charging position of the control valve are identical. In addition for reasons of clarity the following discussion mostly uses the terms cooling mode and cooling speed. The cooling mode is merely an example for a non-charging mode and hence the term cooling mode can be replaced with the more general term non-charging mode.
In the cooling position of the control valve the charging pump, beside the cooling pump, is also fluidly connected with the low-pressure circuit and decoupled from the high-pressure circuit. In the cooling mode the hydraulic pumps operate—in contrast to the charging mode—at a lower pump load with correspondingly lower actual power consumption.
In the state of the art a requirement for charging the pressure accumulator is detected via a sensor, for example a pressure sensor arranged in the high-pressure circuit. As soon as the accumulator pressure falls below a predetermined lower threshold value the above-mentioned control valve automatically assumes its charging position. In addition the control device recognizes the requirement for charging the pressure accumulator by means of the sensor. When recognizing the requirement for charging the pressure accumulator, the control device controls the common electric motor to run at the charging setpoint speed, thereby charging the pressure accumulator in the high-pressure circuit. As soon as the accumulator pressure exceeds an upper threshold value, the above-mentioned control valve automatically assumes its cooling position. In addition the control device recognizes by means of the sensors that a requirement for charging the pressure accumulator no longer exists.
The arrangement of a sensor (for example a pressure sensor or valve position sensor) in the high-pressure circuit for recognizing a requirement for charging the pressure accumulator increases component costs. In addition the operation of the pressure sensor may be subject to malfunctions.